Continuous processes for the manufacture of composite materials, such as continuous profiles, tapes and flat structures in which continuous fibres, for example glass fibre rovings, carbon fibre rovings or aramide fibre rovings, are drawn through a melt of a thermoplastic plastic, are known.
Hence, a process is described in European Application 0 056 703 in which reinforcing fibre rovings are drawn through a melt of a thermoplastic plastic, into which at least one heated surface is immersed to spread the roving. The materials thus manufactured should have longitudinal bending moduli of at least 70%, preferably at least 90%, of the theoretical value.
In practice, several spreading devices are always required. However, the take-off forces to be applied increase sharply with the number of spreading devices (where cylindrical spreading devices are used with an increasing total looping angle), with the viscosity of the melt and with the take-off rate. As large take-off forces damage the reinforcing fibres and hence diminish the properties of the composite material, the available "working range" is very small. In addition, the quality of impregnation and hence also the quality of the products (in the bending test) decrease with increasing viscosity of the melt and increasing take-off rates. The process of European Application 0 056 703 therefore only produces good results at melt viscosities up to 30 Pas and at low take-off rates (below 0.3 metres/minute). Thus, in the case of most thermoplastic materials, those which are of major interest, namely the high molecular types, cannot be used and in addition, only uneconomically low processing rates are possible.
Although the use of rotating surfaces for spreading the rovings reduces the take-off forces, any broken fibres generally contained in the rovings, adhere to the rotating parts and accumulate thereon so that the impregnating process comes to a standstill after a short time.
It is also not possible to avoid the difficulties mentioned by using a further, similarly known process, in which the molten thermoplastic is introduced directly into a gap between the reinforcing fibre roving and the spreading surface.
In addition, processes are known which operate with transverse spraying heads. Such processes have been widely used for a long time in cable coating. In these processes, thermoplastic material is applied from a melt extruder to a reinforcing fibre strand which is usually moved vertically to the direction of the melt. Processes of this kind which in some cases use specially modified tools are described in U.S. Pat. No. 3,993,726, U.S. Pat. No. 4,588,538, European Patent 0 035 974 and European Patent 0 117 098.
In transverse spraying heads of this type, increased pressure usually occurs over a short section of the reinforcing fibre strand as a result of the conveying pressure of the extruder. Accordingly, this increased pressure only acts on the reinforcing fibre strand for a short period. In the state of the art, the significance of pressure therefore receives varied evaluation: whereas U.S. Pat. No. 3,993,726 refers to the requirement of "high pressure," U.S. Pat. No. 4,588,538 only uses the term "under pressure," whereas European Application 0 035 974 makes no mention at all of a conveying effect of the pressure in the transverse spraying head.